Patent literature 1 describes a method and a device for balancing a load distribution of parallel-connected power semiconductor modules in a converter performing a voltage conversion. The time of turning-on and turning-off of a switch of each module is set such that uniform current loads are obtained in all the modules during switching. Specifically, an actual current value of each module is measured as an actual load value, an average value of the measured actual current is set as a desired current value with respect to each module, and a level of a gate signal sent to each switch is increased or decreased based on a difference of the desired current value and the actual current value of each module. The technology described in patent literature 1 is described also in patent literatures 2 to 4.